1. Field of the Invention
The present invention relates to a parallel operation inverter apparatus, and, more particularly, to a parallel operation inverter apparatus capable of, in parallel, operating an inverter, which performs a switching operation at a high frequency, and another AC power source, and an operating method therefor.
2. DESCRIPTION OF THE PRIOR ART
A conventional parallel operation system for an inverter has been arranged as disclosed in "The Power Converter of Semiconductor" (p.p. 216 to 217, March 1987; The Institute of Electrical Engineers of Japan) such that the time average of the active power and the reactive power which circulate between the inverter and another AC power source is detected to use it to control the phase and the level of the output voltage from the inverter so as to make the above-described active power and the reactive power approximate to zero. The power detection method thus-constituted involves a detection time delay of several tens of ms due to its principle. However, since the conventional inverter having a sufficiently high switching frequency displays a satisfactorily large impedance of the output filter, the excess electric current of the inverter can be controlled even if the execution of the control of the output voltage from, the inverter is delayed due to the delay time which has takes place in the power detection. Therefore, a stable parallel operation can be performed.
However, with progress of the semiconductor devices for use in a power source, there has been a desire to reduce the size of the inverter by raising its switching frequency. Therefore, the impedance of its output filter has been reduced recently. As a result, in a case where an inverter which performs the switching operation at a frequency of 10 kHz or higher is operated in parallel with another AC power source, a problem takes place in that an excess electric current undesirably passes through the inverter due to a slight difference in their output voltage levels. Therefore, the above-described method in which the phase and the level of the output voltage are controlled after the average of the above-described active power and the like has been detected cannot be employed.